The superior mechanical and strength-to-weight properties of carbon fibers and carbon filaments has led to an important class of high performance carbon reinforced composites. These high performance composites have many uses, some of which are for the production of aircraft and automobile body parts for which both strength and light-weight are critical. Such composites enable manufacturers to produce relatively light weight structures without sacrificing strength. Consequently, much research has been directed to producing carbon reinforced composite materials with ever increasing high performance properties and physical features that make them more valuable in commercial products and processes.
While high performance carbon fibers have met with a degree of commercial success as reinforcing materials in fiber/matrix composites, they nevertheless suffer from, inter alia, delamination problems. Several attempts have been made to modify carbon fibers to improve their interlaminar properties. For example, U.S. Pat. No. 4,816,289 teaches a method of producing crimped fibers. While crimped fibers have improved interlaminar properties, they would nevertheless still suffer from an unacceptable degree of delamination.
Another process teaches the formation of graphite fibers onto which secondary silicon carbide whiskers can be grown. While such a structure would show a substantial improvement in interlaminar shear strength, they unfortunately suffer from a number of shortcomings. For example, in the production of silicon carbide whiskers,. relatively high temperatures (i.e., &gt;1000.degree. C.) are required. Still further, silicon carbide is intrinsically abrasive and thus leads to handling and processing problems. Other shortcomings include: (a) the thermal expansion coefficient of silicon carbide differs from that of carbon, and as a consequence, can initiate or propagate cracks in the resulting composite; (b) the densities of silicon carbide whiskers(ca. 3.22 g/cc) are considerably higher than those of carbon (2.25 g/cc); (c) at present, it is not possible to control silicon carbide whisker orientation and growth characteristics; and (d) the costs associated with producing secondary silicon carbide whiskers on carbon fibers is considerable. It is also believed that the bonding between the silicon carbide whiskers and the parent carbon fibers is non-chemical and thus would not be as strong as desired in certain applications.
Another process variation is taught in Sekiyu Gakkaishi, 28(5), 409-412, Egashira et. al, 1985, wherein carbon whiskers are grown on carbon fibers from the vapor phase catalyzed by iron sulfide. For example, the parent fibers are preoxidized with HNO.sub.3 at a temperature of about 120.degree. C. for one hour to facilitate supporting Fe on them. They are then impregnated with a 0.5 mol/L Fe(NO.sub.3).sub.3 solution, followed by reduction with hydrogen. A mixture of benzene, H.sub.2 S, and H.sub.2 are employed as the reactant gas. The whiskers, or filaments, produced are straight, non-branched filaments. That is, they cannot be characterized as being branched, spiral, or helical, as are the filaments of the present invention.
U.S. Pat. No. 5,149,584, to Baker et al. teaches carbon fiber/carbon filament structures in a matrix material, such as polymers, carbon, and ceramics. Also, co-pending application U.S. Ser. No. 07/947,416, filed Sep. 18, 1992, now U.S. Pat. No. 5,413,866, claims carbon filaments, some of which are used in the present invention.
While such methods of modifying carbon fibers do improve the interlaminar shear strength of the parent fibers to various degrees, there still exists a need in the art for high performance composite structures reinforced with carbon filaments alone, particularly those that are conductive. There is a growing demand for composite materials which have electrical conductive properties or which can act as insulators. Consequently, there is a need in the art for high performance composite materials containing superior reinforcing components and which can also conduct electricity.